This invention relates to the manufacture of draw tape for thermoplastic bags, and more particularly, to making an intermittently stretched and oriented draw tape.
Bags made of thin polyethylene material have been used in various sizes. Small bags are used in the packaging of sandwiches and the like; larger bags are used as shopping bags; and even larger bags are used for containing trash.
A particularly advantageous closure for such a bag includes a draw tape constructed from the same polyethylene material. U.S. Pat. No. 3,029,853--Piazzi, and British Pat. No. 1,125,363--Jortikka are examples of draw tape bags. Such closures have been successfully employed on these bags.
Draw tape closures for large trash bags, and the manufacture of these draw tape trash bags, are described in the related applications identified below.
In such bags, the tape is preferably unoriented polyethylene which is heat sealed to the opposing panels of the bag at the sides thereof. The weakest part of the draw tape is at the heat seal. This weak point should be located at the point of lowest tensile load, which is furthest from where the user pulls the tape. For this reason, the tape is normally in a hem with a notch at the center of the bag, so that the user grasps the tape at a point furthest from the heat seal.
Economy of manufacture is one of the prime considerations in these bags. The cost of the draw tape is a significant part of the total bag cost, so that any economies in draw tape cost advantageously reduce the ultimate cost of the bag.
As more fully set forth in the Herrington application identified below, an improved draw tape for trash bags of this type has a stretched relatively thin central portion between unstretched, relatively thick and wide end portions of the tape. The end portions are heat sealed at the sides of the bag. This stretched tape has an economic advantage and improved load carrying characteristics.
It is an object of the present invention to provide a method of and apparatus for intermittently stretching thermoplastic tape to produce such draw tapes for bags.
It is another object of the invention to provide tape stretching apparatus which can be easily synchronized with thermoplastic bag making machinery.
It is another object of the present invention to provide stretching apparatus in which the stretching can take place over a period of time that is greater than the cycle time for the bag machine.